Method and system for maintaining jetting stability in a jetting device

ABSTRACT

The invention relates to a method for maintaining and/or restoring the jetting stability in a jetting device, the jetting comprising fluid chamber body having arranged therein an orifice, the jetting device being configured to comprise a quantity of an electrically conductive fluid. The jetting device comprises actuation means, comprising a magnetic field generating means and an electrical current generating means for, in operation, applying an actuation pulse to the electrically conductive fluid. The method for maintaining and/or restoring the jetting stability comprises applying an maintenance pulse to at least a part of the electrically conductive fluid. The invention further relates to a jetting device, employing the described method.

The present invention relates to a method and a system for jettingelectrically conductive fluids and more in particular to a method and asystem for maintaining jetting stability in said system.

BACKGROUND OF THE INVENTION

A jetting device for ejecting droplets of an electrically conductivefluid, such as a molten metal or a molten semiconductor is known. Anexample of a jetting device for ejecting droplets of an electricallyconductive fluid is described in WO 2010/063576 A1. In such a printingdevice, a Lorentz force is generated in the electrically conductivefluid due to which a droplet is expelled through an orifice nozzle ofthe printing device. Such a device may be used for ejecting droplets ofa fluid having a high temperature, for example a molten metal having ahigh melting point.

Direct printing of molten metals may be employed for printing electroniccircuitry, for example. In such an application it is essential that alldroplets are actually printed accurately as otherwise the electroniccircuitry may not function due to an interruption in the electronicconnections as a result of a missing droplet, for example. Therefore, itis desirable that all droplets are actually generated. Thus, jettingstability has to be maintained. However, the jetting stability maydecrease during the jetting process, for example by (partial) blockingof an orifice. An orifice may be blocked, for example by impurities thathave build up in the orifice, or by (partial) solidification of theelectrically conductive fluid in the proximity of the orifice. When anorifice is blocked, it may be more difficult or even impossible to ejecta droplet of fluid from the orifice. As a consequence, a decrease in thejetting stability may therefore result in missing droplets.

It is therefore desirable to maintain or—if necessary—restore thejetting stability. It is known to restore the jetting stability of ajetting device by purging the orifice. The orifice may be purged byapplying a purge pulse. U.S. Pat. No. 4,245,224 discloses apiezoelectric jetting device for jetting droplets of ink, wherein thejetting stability may be restored by applying a purge pulse. The purgepulse may apply a larger force to the fluid than a regular actuationpulse and/or the purge pulse may be maintained longer than a regularactuation pulse. Also in jetting devices for jetting an electricallyconductive fluid, wherein the fluid is jetted using Lorentz actuation, apurge pulse may be applied. However, it was found that applying apositive purge pulse does not always result in restoring the jettingstability in a device for jetting an electrically conductive fluid.

It is an object of the invention to provide a method for maintaining thejetting stability of a jetting device.

SUMMARY OF THE INVENTION

The above object is achieved in a method for maintaining jettingstability in a jetting device for jetting a droplet of an electricallyconductive fluid, the jetting device comprising a fluid chamber bodydefining a fluid chamber and having an orifice extending from the fluidchamber to an outer surface of the fluid chamber element and anactuation means, the actuation means comprising:

-   -   a magnetic field generating means for generating a magnetic        field in at least a part of the fluid chamber; and    -   an electrical current generating means for generating an        electrical current in the electrically conductive fluid in the        part of the fluid chamber provided with the magnetic field,    -   the actuation means being configured to provide an actuation        pulse for expelling droplets of the electrically conductive        fluid from the fluid chamber through the orifice, the actuation        means being further configured to provide a maintenance pulse,        the actuation pulse and the maintenance pulse each generating a        Lorentz force in the conductive fluid in said part of the fluid        chamber,    -   the method comprising the step of:    -   a) applying the maintenance pulse to at least a part of the        electrically conductive fluid in the part of the chamber        provided with the magnetic field, the maintenance pulse being        configured to retract a meniscus of the electrically conductive        fluid into the fluid chamber.

In a known system for printing an electrically conductive fluid, adroplet of said electrically conductive fluid is expelled through anorifice by a Lorentz force. This force causes a motion in the conductivefluid. This motion may cause a part of the fluid to move from the fluidchamber through the orifice, thereby generating a droplet of the fluid.The Lorentz force is related to the electric current and the magneticfield vector; {right arrow over (F)}={right arrow over (I)}×{right arrowover (B)}. The Lorentz force resulting from the electric current and themagnetic field is generated in a direction perpendicular to both theelectrical current and the magnetic field. By suitably selecting thedirection and the magnitude of the electric current, as well as thedirection and the magnitude of the magnetic field, the direction and themagnitude of the resulting Lorentz force may be selected. In the systemaccording to the present invention, in normal operation, the magneticfield is provided and an electrical current is provided in theconductive fluid, such that a suitable force for ejecting a droplet isgenerated.

The jetting device in accordance with the present invention comprises afluid chamber and has an orifice extending from the fluid chamber to anouter surface of the fluid chamber element. In operation, the fluidchamber comprises an electrically conductive fluid. The electricallyconductive fluid may be a molten metal or a molten semiconductor. Inaddition, the fluid may be a mixture of molten metals, a mixture ofmolten semiconductors or a mixture of at least one molten metal and atleast one molten semiconductor. For example, droplets of molten silver,molten gold, molten copper or molten solder may be jetted using thejetting device in accordance with the present invention. Theelectrically conductive fluids may be essentially free of solvents;thus, the metal or semiconductor does not need to be dissolved, but maybe jetted in its essentially pure (molten) form. If the fluid isessentially free of solvents, no changes in composition of the fluid mayoccur due to evaporation of the solvent. As a consequence, thecomposition of the fluid in the fluid chamber, as well as itsproperties, may not change with time.

When applying an actuation pulse, a Lorentz force is generated withinthe fluid, causing the fluid to move through the orifice in a directionaway from the fluid chamber. The actuation pulse may be applied byapplying a pulsed magnetic field and a continuous electrical current, ora pulsed electrical current in a continuous magnetic field, or acombination thereof. Alternatively, a constant Lorentz force may begenerated within the fluid by applying a constant electrical current tothe electrically conductive fluid in a constant magnetic field. However,application of a constant Lorentz force to the electrically conductivefluid may result in the ejection of a stream of the electricallyconductive fluid, instead of in the ejection of droplets.

The actuation pulse, provided by a pulse of electrical current or apulse of a magnetic field, or both, may have any shape or magnitude,provided that the actuation pulse is suited to, in normal operation ofthe jetting device, provide a force in the electrically conductive fluidthat is sufficient to eject a droplet of the fluid through the orifice.Various types of actuation pulses are known in the art. Optionally, anactuation pulse may comprise a plurality of sub pulses. For example,from U.S. Pat. No. 5,377,961 it is known to actuate an electricallyconductive fluid by applying positive and negative actuation pulses tothe fluid, wherein the positive pulses and the negative pulsesinterchange. It is described that the positive pulse serves to move aportion of the fluid through the orifice and that the negative pulseserves to move a part of the fluid back towards the fluid reservoir,thereby forming a droplet. Thus, by suitable composing the actuationpulse from a plurality of sub-pulses, a droplet of a suitable size maybe ejected. Moreover, other effects, such as satellite droplets may beprevented by suitably composing the actuation pulse. In any case, theactuation pulse should be composed such, that the actuation pulse, innormal operation, provides a net force to the electrically conductivefluid to move through a nozzle away from the fluid chamber.

Also the maintenance pulse may be suitably composed, e.g. from a singlenegative pulse or from a plurality of sub pulses. By suitably composingthe maintenance pulse, the movement of the electrically conductive fluidpositioned within the magnetic field as well as the movement of themeniscus of the electrically conductive fluid may be suitablycontrolled. The maintenance pulse may be composed such that the meniscusof the electrically conductive fluid is retracted upon applying themaintenance pulse. The maintenance pulse may preferably be composed suchthat no droplet of fluid is expelled upon applying the maintenancepulse.

As mentioned above, the jetting stability may decrease during thejetting process. This may result e.g. in droplets not having the desiredsize being jetted, droplets not being jetted at a desired jetting angle,or even no droplets being jetted at all upon applying an actuationpulse. In that case, it may be desirable to restore the jettingstability. As describes above, a maintenance pulse may be applied torestore the jetting stability. Preferably, the maintenance pulse may beapplied by the same actuation means that apply the actuation pulse tothe electrically conductive fluid. Alternatively, separate actuationmeans, configured to provide a maintenance pulse, may be provided, theseactuation means being configured to apply a maintenance pulse to atleast a part of the electrically conductive fluid in the part of thechamber provided with the magnetic field. The maintenance pulse may be asingle pulse or may comprise a plurality of sub pulses. The sub pulsesmay be positive sub pulses, negative sub pulses, or a combinationthereof. The maintenance pulse may be the inverse of the actuationpulse, but this is not necessary. The maintenance pulse generates aforce in the electrically conductive fluid that is directed oppositewith respect to the force generated in the electrically conductive fluidby the actuation pulse. The maintenance pulse therefore generates aforce in the electrically conductive fluid that is directed in adirection from the orifice of the fluid chamber body to the fluidchamber. When the maintenance pulse is applied to the electricallyconductive fluid, and this fluid is positioned in the magnetic field andis in electrically conducting contact with the electrical currentgenerating means, then the fluid experiences a Lorentz force that movesthe fluid from the orifice into the fluid chamber. Therefore, a meniscusof the electrically conductive fluid may be retracted into the fluidchamber upon application of the maintenance pulse. In addition, themaintenance may be applied without a droplet being subsequentlyexpelled.

Moreover, because of the force applied to the electrically conductivematerial, any conductive material, such as the electrically conductivefluid, but also particles of the fluid that have solidified, as well aselectrically conductive contaminant present in the vicinity of theorifice may be moved away from the orifice to the fluid chamber body.Moreover, also non-electrically conductive material present in thevicinity of the orifice, may be moved away from the orifice to the fluidchamber body, together with the electrically conductive fluid uponapplying an maintenance pulse. Consequently, the application of amaintenance pulse to the electrically conductive may remove impuritiesin the orifice region that hamper the jetting process. When there is nomore material present in the vicinity of the orifice -besides theelectrically conductive fluid—the jetting stability may be restored. Inthis way, applying a maintenance pulse may restore the jettingstability. The maintenance pulse may be applied to the electricallyconductive fluid at regular intervals to maintain the jetting stabilityof the jetting device. Alternatively, the maintenance pulse may beapplied to the electrically conductive fluid upon detection of acondition of the jetting device, for example upon detection ofmalfunctioning of the jetting device, for example blocking of theorifice.

A single maintenance pulse may be applied to the electrically conductivefluid or a sequence of a plurality of maintenance pulses may be appliedto the electrically conductive fluid. In the latter case, optionally thecondition of the jetting device may be monitored after each maintenancepulse or after a set of maintenance pulses, wherein, depending of thecondition of the jetting device, more maintenance pulses may be appliedor not.

Please note that it may be possible, depending on the design of thefluid chamber body, to control the maximum distance between the orificeand the electrically conductive fluid that may arise from applying themaintenance pulse. When the electrically conductive fluid, and possiblyalso other electrically conductive material that is present around theorifice, moves away from the orifice into the fluid chamber, than it maymove to a position in the fluid chamber body where it is no longer inelectrically conductive contact with the electrical current generatingmeans and/or where it is no longer positioned within the magnetic field.At this point, there will be no more Lorentz force resulting from themaintenance pulse acting within the fluid and there is no more drivingforce which causes the fluid to move away from the orifice. Therefore,the fluid may not move too far away from the orifice.

When a maintenance pulse configured to retract a meniscus of theelectrically conductive fluid into the fluid chamber is applied to theelectrically fluid, and the meniscus is retracted, the retraction of themeniscus may result in an air bubble entering the fluid chamber. Thepresence of air bubbles may give problems regarding jetting stability inknown methods for actuating fluids, for example in piezoelectricactuators. However. in case the actuator is a Lorentz actuator, thepresence of air may not have the same negative effect on the jettingprocess as described with respect to the piezoelectric actuator. Indevices for jetting an electrically conductive fluid using Lorentzactuation, a force is generated in the form of motion within the fluiditself, provided the fluid is an electrically conductive fluid. Thus, inLorentz actuation, the Lorentz force generated acts directly on thefluid, without a pressure wave having to be build up. Therefore, even incase an air bubble is formed in the fluid chamber, for example byapplying a maintenance pulse, fluid may still be jetted. Moreover, sincethe flow of the fluid does not need to be restricted, as for example ina piezoelectric actuator, to be able to eject a droplet, a Lorentzactuator may be constructed such that an air bubble entered in the fluidchamber may easily escape, for example by allowing the air bubble tofloat upwards towards a position outside of the fluid chamber. Theescape of an air bubble from the fluid chamber may be facilitated byapplying a maintenance pulse, or a series of maintenance pulses, to theelectrically conductive fluid. The movement of the fluid away from theorifice, as a result of the maintenance pulse, may induce a movement ofthe air bubble away from the orifice.

It is also possible to restore the jetting stability of a jettingdevice, which jets droplets of a fluid using Lorentz actuation byapplying a positive purge pulse. A positive purge pulse is a pulse thathas a higher amplitude and/or a longer pulse width than an actuationpulse. The positive purge pulse provides a larger Lorentz force to theelectrically conductive fluid than an actuation pulse. Therefore,contaminants may be pushed away from the vicinity of the orifice. Whenthe contaminants have been removed, jetting stability may be restored.However, the application of the purge pulse may not always result inrestoring the jetting stability.

In addition, applying a positive purge pulse may result in the ejectionof contaminants and/or a relatively large amount of fluid, which may endup on the receiving material, thereby negatively influencing theprinting quality, when the positive purge pulse is applied during aprint job. The positive purge pulse may be applied in between printjobs. However, if the jet stability decreases during a print job, it isdesirable to be able to restore the jetting stability immediately,without having to wait for the print job to be finished. Thesedisadvantages are mitigated by applying a maintenance pulse, such as anegative purge pulse, instead of a positive purge pulse. Alternatively,it is possible to restore the jetting stability of a jetting device byapplying a combination of positive and negative purge pulses.

In an embodiment the maintenance pulse is provided by generating aninverse electrical current in the electrically conductive fluid in thepart of the chamber provided with the magnetic field. As stated above,the direction and the magnitude of the resulting Lorentz force may besuitably selected by suitably selecting the direction and the magnitudeof the electric current, as well as the direction and the magnitude ofthe magnetic field. Thus, the direction of the Lorentz force generatedin the electrically conductive fluid may be inverted by inverting thedirection of the electrical current applied to the electricallyconductive fluid and leaving the direction of the magnetic fieldunchanged. The actuation means may be used to apply the maintenancepulse to the system. In this embodiment, the electrical current may beinverted, e.g by changing the direction of the current generated by theelectrical current generating means.

In an embodiment, the maintenance pulse consists of a single negativepulse. A single negative pulse may suffice to restore the jettingstability.

In an embodiment, a series of maintenance pulses, each of themaintenance pulses consisting of a single negative pulse, may beapplied.

In an embodiment, the method further comprises the steps of:

-   -   b) detecting a resulting electrical current, which electrical        current is induced by a residual force in the part of the        conductive fluid positioned in the magnetic field, thereby        obtaining a detection signal ; and    -   c) based on the detection signal determining whether the jetting        device is in an operative state,

wherein steps b) and c) are performed after providing an actuationpulse, and wherein, if the jetting device is not in an operative state,step a) is performed.

In the jetting device for jetting a droplet of an electricallyconductive fluid in accordance with the present invention, a droplet ofsaid electrically conductive fluid is expelled through an orifice by aLorentz force. This force causes a motion in the conductive fluid. TheLorentz force is related to the electric current and the magnetic fieldvector; {right arrow over (F)}={right arrow over (I)}×{right arrow over(B)}. In the system according to the present invention, the magneticfield is provided and an electrical current is provided in theconductive fluid, such that a suitable force for ejecting a droplet isgenerated. Before a droplet of fluid is ejected, a motion has beengenerated in the electrically conductive fluid by the Lorentz force. Dueto inertia, the motion within the fluid in the fluid chamber afterejection of the droplet, does not disappear momentarily as soon as theapplication of the electrical current is stopped, but will graduallyfade in the course of time. The residual motion of the fluid in thefluid chamber as a function of time will depend, amongst others, on theacoustic behavior of the fluid chamber. The motion in the conductivefluid generates a force. Thus, after ejection of a droplet, a force isgenerated in the fluid. Since the conductive fluid is positioned in amagnetic field, an induced current ({right arrow over (I)}) is generatedin the fluid, because of the relation {right arrow over (F)}={rightarrow over (I)}×{right arrow over (B)}. By measuring this current,hereafter referred to as resulting electric current, a detection signalmay be obtained. Based on the detection signal, the acoustics in theactuation chamber may be monitored. A method for monitoring aperformance of a jetting device configured to expel droplets of anelectrically conductive fluid is described in more detail in WO2011/113703 A1.

However, also in the case that providing an electrical current in thepresence of a magnetic field does not lead to the ejection of a droplet-or in case a droplet is generated that deviates from a normal droplet,e.g. in size or in jetting angle—still the acoustic behavior of thefluid may be monitored. As explained above, applying an electricalcurrent through a conductive fluid in a magnetic field generates aLorentz force, which results in a residual motion in the conductivefluid that will gradually fade in the course of time. The residualmotion as a function of time will depend, amongst others, on theacoustic behavior of the fluid chamber. The acoustic behavior includesinter alia the resonances due to the shape of the fluid chamber and dueto the presence or absence of fluid and due to the presence or absenceof impurities in the vicinity of the orifice, such as solid particles.Since a magnetic field is applied to the conductive fluid, the forceresulting from the residual motion will induce a current through thefluid, in accordance with {right arrow over (F)}={right arrow over(I)}×{right arrow over (B)}, which may be sensed by suitable means. Bymeasuring this detection signal, the acoustic behavior of the actuationchamber may be monitored and as a consequence, it may be determinedwhether the jetting device is in an operative state or not.

If the jetting device is not (anymore) in an operative state, it may bedesired to restore the jetting stability. Restoring the jettingstability may be performed by applying a maintenance pulse to at least apart of the electrically conductive fluid in the part of the chamberprovided with the magnetic field. By applying the maintenance pulse, thejetting stability may be restored, e.g. by removing impurities from thevicinity of the orifice that were hampering the jetting process.

The steps of detecting a resulting electrical current, thereby obtaininga detection signal and determining, based on the detection signal,whether the jetting device is in an operative state or not, may becarried out after applying an actuation pulse. For example, these stepsmay be carried out during a print job. As mentioned above, applying amaintenance pulse does not result in the jetting of a droplet andtherefore, the print result may not be negatively influenced. Inaddition, by monitoring the jetting stability, it is possible to restorejetting stability before device starts malfunctioning. Consequently, alldroplets of the print job may be printed accurately, resulting in goodprint quality.

Steps b) and c) may be carried out continuously, or alternatively, theymay be carried out at less frequently, for example in between print jobsor after a plurality of print jobs.

In an aspect of the invention, the present invention further provides ajetting device for jetting a droplet of an electrically conductivefluid, the jetting device comprising:

-   -   a fluid chamber body defining a fluid chamber and having an        orifice extending from the fluid chamber to an outer surface of        the fluid chamber element, the fluid chamber being configured        for holding an amount of the electrically conductive fluid; and    -   an actuation means configured to provide an actuation pulse for        expelling droplets of the electrically conductive fluid from the        fluid chamber through the orifice, the actuation means        comprising:    -   a magnetic field generating means for generating a magnetic        field in at least a part of the fluid chamber; and    -   an electrical current generating means for generating an        electrical current in the electrically conductive fluid in the        part of the fluid chamber provided with the magnetic field,    -   the actuation means being further configured to provide a        maintenance pulse for retracting the meniscus of the        electrically conductive fluid into the fluid chamber.

The jetting device according to the present invention is thus configuredfor performing the method according to the present invention.

The actuation means may be efficiently embodied as being configured toprovide both the actuation pulse for expelling droplets of theelectrically conductive fluid from the fluid chamber through theorifice, an the maintenance pulse for retracting the meniscus of theelectrically conductive fluid into the fluid chamber. Alternatively,means for providing the actuation pulse and means for providing themaintenance pulse not be embodied together. In that case, both theactuation means and the means for applying the maintenance pulse maycomprise electrical current generating means and magnetic fieldgenerating means.

BRIEF DESCRIPTION OF THE DRAWINGS

These and further features and advantages of the present invention areexplained hereinafter with reference to the accompanying drawingsshowing non-limiting embodiments and wherein:

FIG. 1 shows a perspective view of a printing device for printingdroplets of an electrically conductive fluid.

FIG. 2 shows a cross-sectional view of a part of the printing deviceshown in FIG. 1.

FIG. 3A and FIG. 3B show a number of examples of actuation pulses.

FIG. 4A and FIG. 4B show a number of examples of maintenance pulses.

In the drawings, same reference numerals refer to same elements.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a part of a jetting device 1 for ejecting droplets of anelectrically conductive fluid, in particular a molten metal such ascopper, silver, gold and the like. The jetting device 1 comprises asupport frame 2. Molten metals such as copper, silver, gold, or moltensemiconductors are generally materials having a high melting point.Therefore, in the molten state, such molten metals or semiconductors maybe relatively hot fluids. Therefore, the support frame 2 is preferablymade of a heat resistant and heat conductive material.

The jetting device 1 is provided with an ejection orifice 4 throughwhich a droplet of the fluid may be ejected. The orifice or nozzle 4 isa through hole extending through a wall of a fluid chamber body 6. Inthe fluid chamber body 6 a fluid chamber is arranged. The fluid chamberis configured to hold the electrically conductive fluid.

For ejecting droplets of the electrically conductive fluid, the jettingdevice 1 is provided with two permanent magnets 8 a, 8 b (hereinafteralso referred to as magnets 8). The magnets 8 are arranged between twomagnetic field concentrating elements 10 a, 10 b (hereinafter alsoreferred to as concentrators 10) made of magnetic field guiding materialsuch as iron. The jetting device 1 is further provided with twoelectrodes 12 a, 12 b (hereinafter also referred to as electrodes 12)both extending into the fluid chamber body 6 through a suitable throughhole such that at least a tip of each of the electrodes 12 is in directelectrical contact with the molten metal present in the fluid chamber.The electrodes 12 are supported by suitable electrode supports 14 andare each operatively connectable to a suitable electrical currentgenerator (not shown) such that a suitable electrical current may begenerated through the electrodes 12 and the molten metal present betweenthe tips of the electrodes 12. The electrodes 12 may be each operativelyconnectable to an electrical signal detection unit (not shown), suchthat a resulting current, induced by a residual pressure wave in thepart of the fluid positioned in the magnetic field, may be monitored.

In the jetting device 1 as shown in FIG. 1, the magnets 8, theconcentrators 10 and the electrodes 12 are configured to apply anactuation pulse to the electrically conductive fluid, as well as toprovide a maintenance pulse to the fluid.

FIG. 2 shows a cross-section of the embodiment illustrated in FIG. 1,which cross-section is taken along line b-b (FIG. 1). Referring to FIG.2, the support frame 2 and the magnets 8 are shown. In the illustratedembodiment, the support frame 2 is provided with cooling channels 34through which a cooling liquid may flow for actively cooling of thesupport frame 2 and the magnets 8. An induction coil 24 is shown. Thefluid chamber body 6 is arranged in a centre of the induction coil 24such that a current flowing through the induction coil 24 results inheating of a metal arranged in the fluid chamber 6. Due to such heatingthe metal may melt and thus become a fluid. Such inductive heatingensures a power-efficient heating and no contact between any heatingelement and the fluid, limiting a number of (possible) interactionsbetween elements of the jetting device 1 and the fluid. Nevertheless, inother embodiments, other means for heating the metal, or anotherelectrically conductive fluid, in the fluid chamber may be applied.

The fluid chamber body 6 of the jetting device as depicted in FIG. 2 hasan open connection 35 to the environment at the top of the fluid chamberbody. Because of this open connection, air bubbles or gas bubbles thatmay have entered the fluid chamber 23, for example air or gas bubblesthat have entered the fluid chamber 23 upon applying an maintenancepulse, may leave the fluid chamber 23 and the fluid chamber body 6 viathe open connection 35 to the environment.

FIG. 3A-3B show a number of examples of actuation pulses as may begenerated by an actuation means in a conductive fluid in a fluid chamberbody. FIG. 3A shows an actuation pulse P₁ that consists of a singlepositive pulse. The actuation pulse is the magnitude of {right arrowover (I)}×{right arrow over (B)} as a function of time, during the pulselength. The total pulse length of the actuation pulse P₁ is Δt₁+Δt₂+Δt₃.The total pulse length Δt₁+Δt₂+Δt₃ may be in the range of 2 μs to 250μs. During a first period of time Δt_(t), the amplitude of the pulsegradual increases, thereby applying a gradually increasing force ({rightarrow over (F)}={right arrow over (I)}×{right arrow over (B)}) to theelectrically conductive fluid, until the maximum amplitude A₁ isreached. During a second period of time Δt₂, the magnitude of the forceapplied to the electrically conductive fluid is constant and has amagnitude A₁. After the expiry of the second period of time Δt₂, thereis a third period of time Δt₃, wherein the magnitude of the forceapplied to the electrically conductive fluid gradually decreases untilit becomes zero. The length of the periods of time Δt₁, Δt₂, Δt₃ mayvary. In an alternative embodiment, the actuation pulse may be a stepfunction. In that case, the first and third periods of time Δt₁, Δt₃ arezero are almost zero.

FIG. 3B shows an actuation pulse P₂ that consists of a plurality of subpulses. During a period of time Δt₁+Δt₂+Δt₃, a first sub pulse isapplied to the electrically conductive fluid. This first sub pulse is apositive sub pulse during which a positive force is applied to theelectrically conductive fluid. This positive force may result in theejection of a part of the fluid through the orifice. After the first subpulse has been applied to the fluid, during a fourth period of time Δt₄,there is an optional pause in the actuation pulse, during which no forceis applied to the fluid by the actuation means. However, a residualforce, resulting from the Lorentz force applied to the electricallyconductive fluid may be present and cause motions within the fluidduring the fourth period of time Δt_(t). The pause in the actuationpulse is optional; therefore, the fourth period of time may be 0. Duringa period of time Δt₆ a second sub pulse is applied to the system. Thesecond sub pulse is a negative sub pulse during which a negative forceis applied to the electrically conductive fluid. The negative sub pulseis shown as a step shaped pulse. However, the negative sub pulse mayhave any suitable shape. For example, the decrease and/or the increaseof the force applied to the fluid may be gradual. The negative sub pulsegenerates a force in the electrically conductive fluid having adirection opposite with respect to the force applied to the fluid by thepositive sub pulse. This results in the retraction of (a part of thefluid) into the fluid chamber. This may be used for example to controlthe size of a droplet ejected by the jetting device.

Finally, during a period of time Δt₆+Δt₇, a third sub pulse is appliedto the fluid, the third sub pulse being a positive sub pulse. The thirdsub pulse, as depicted in FIG. 3B shows a instantaneous incline to apulse having a magnitude corresponding to the amplitude Δ₃. After asixth period of time Δt₆ wherein a constant force is applied to thefluid, the force gradually decreases to 0 during a seventh period oftime Δ₇. The third sub pulse may be applied to the fluid, for example tostabilise a meniscus of the fluid.

FIG. 4A and FIG. 4B show a number of examples of maintenance pulses asmay be generated by an actuation means in a conductive fluid in a fluidchamber body. FIG. 4A shows an maintenance pulse P₃ that consists of asingle negative pulse. The total pulse length of the actuation pulse P₁is Δt₁₀+Δt₁₁+Δt₁₂. The total pulse length Δt₁₀+Δt₁₁+Δt₁₂ may be in therange of 5 μs to 250 μs. During an tenth period of time Δt₁₀, theamplitude of the pulse gradually increases, thereby applying a graduallyincreasing negative force ({right arrow over (F)}={right arrow over(I)}×{right arrow over (B)}) to the electrically conductive fluid, untilthe maximum amplitude A₁ is reached. During a eleventh period of timeΔt₁₁, the magnitude of the force applied to the electrically conductivefluid is constant and has a magnitude Δ₁₁. After the expiry of thesecond period of time Δt₁₁, there is a twelfth period of time Δt₁₂,wherein the magnitude of the force applied to the electricallyconductive fluid gradually decreases until it becomes zero.

The length of the periods of time Δt₁₀+Δt₁₁+Δt₁₂ may vary. In analternative embodiment, the actuation pulse may be a step function. Inthat case, the tenth and twelfth periods of time Δt₁₀, Δt₁₂ are zero.During the maintenance pulse, a force is generated in the fluid andoptionally, within electrically conductive contaminant present in thevicinity of the orifice. The pulse may therefore result in theelectrically conductive fluid and/or the electrically conductivecontaminant present in the vicinity of the orifice moving from theorifice into the fluid chamber. In this way, the orifice and itsvicinity may be cleaned from contaminants and the jetting stability maybe restored.

FIG. 4B shows an maintenance pulse P₄ that consists of a plurality ofsub pulses. During a period of time Δt₁₀+Δt₁₁+Δt₁₂, a first sub pulse isapplied to the electrically conductive fluid by the inverse actuationmeans (not shown). The inverse actuation means may be the actuationmeans. This first sub pulse is a negative sub pulse during which anegative force is applied to the electrically conductive fluid. Thisnegative force may result in the movement of (a part of) the fluid fromthe orifice into the fluid chamber. Optionally, contaminants, bothelectrically conductive and non-electrically conductive contaminants, orair bubbles, may move with the fluid away from the orifice into thefluid chamber. After the first sub pulse has been applied to the fluid,during a thirteenth period of time Δt₁₃, there is an optional pause inthe actuation pulse, during which no force is applied to the fluid bythe inverse actuation means. However, a residual force, resulting fromthe Lorentz force applied to the electrically conductive fluid may bepresent and cause motions within the fluid during the thirteenth periodof time Δt₁₃. The pause in the actuation pulse is optional. Therefore,the thirteenth period of time may be zero.

During a period of time Δt₁₄+Δt₁₅+Δt₁₆ a second sub pulse is applied tothe system. The second sub pulse is a positive sub pulse during which apositive force is applied to the electrically conductive fluid. Thepositive sub pulse is shown as a pulse showing a gradual increase and agradual decrease in amplitude. However, the positive sub pulse may haveany suitable shape. For example, the sub pulse may be the shape of astep function. The positive sub pulse generates a force in theelectrically conductive fluid having a direction opposite with respectto the force applied to the fluid by the negative sub pulse. Finally,during a period of time Δt₁₇, a third sub pulse is applied to the fluid,the third sub pulse being a negative sub pulse. The third sub pulse, asdepicted in FIG. 4B shows a instantaneous incline to a pulse having amagnitude corresponding to the amplitude A₁₇. The second and/or thirdsub pulse may be applied to the fluid, for example to stabilise ameniscus of the fluid.

Detailed embodiments of the present invention are disclosed herein;however, it is to be understood that the disclosed embodiments aremerely exemplary of the invention, which can be embodied in variousforms. Therefore, specific structural and functional details disclosedherein are not to be interpreted as limiting, but merely as a basis forthe claims and as a representative basis for teaching one skilled in theart to variously employ the present invention in virtually andappropriately detailed structure. In particular, features presented anddescribed in separate dependent claims may be applied in combination andany combination of such claims are herewith disclosed. Further, theterms and phrases used herein are not intended to be limiting; butrather, to provide an understandable description of the invention. Theterms “a” or “an”, as used herein, are defined as one or more than one.The term plurality, as used herein, is defined as two or more than two.The term another, as used herein, is defined as at least a second ormore. The terms including and/or having, as used herein, are defined ascomprising (i.e., open language).

1. Method for maintaining jetting stability in a jetting device forjetting a droplet of an electrically conductive fluid, the jettingdevice comprising a fluid chamber body defining a fluid chamber andhaving an orifice extending from the fluid chamber to an outer surfaceof the fluid chamber element and an actuation means, the actuation meanscomprising: a magnetic field generating means for generating a magneticfield in at least a part of the fluid chamber; and an electrical currentgenerating means for generating an electrical current in theelectrically conductive fluid in the part of the fluid chamber providedwith the magnetic field, the actuation means being configured to providean actuation pulse for expelling droplets of the electrically conductivefluid from the fluid chamber through the orifice, the actuation meansbeing further configured to provide a maintenance pulse, the actuationpulse and the maintenance pulse each generating a Lorentz force in theconductive fluid in said part of the fluid chamber, the methodcomprising the step of: a) applying the maintenance pulse to at least apart of the electrically conductive fluid in the part of the chamberprovided with the magnetic field, the maintenance pulse being configuredto retract a meniscus of the electrically conductive fluid into thefluid chamber.
 2. Method according to claim 1, wherein the maintenancepulse is provided by generating an inverse electrical current in theelectrically conductive fluid in the part of the chamber provided withthe magnetic field.
 3. Method according to claim 1, wherein themaintenance pulse consists of a single negative pulse.
 4. Methodaccording to claim 1, wherein the method further comprises the steps of:b) detecting a resulting electrical current, which electrical current isinduced by a residual force in the part of the conductive fluidpositioned in the magnetic field, thereby obtaining a detection signal;and c) based on the detection signal determining whether the jettingdevice is in an operative state, wherein steps b) and c) are performedafter providing an actuation pulse, and wherein, if the jetting deviceis not in an operative state, step a) is performed.
 5. Jetting devicefor jetting a droplet of an electrically conductive fluid, the jettingdevice comprising: a fluid chamber body defining a fluid chamber andhaving an orifice extending from the fluid chamber to an outer surfaceof the fluid chamber element, the fluid chamber being configured forholding an amount of the electrically conductive fluid; and an actuationmeans configured to provide an actuation pulse for expelling droplets ofthe electrically conductive fluid from the fluid chamber through theorifice, the actuation means comprising: a magnetic field generatingmeans for generating a magnetic field in at least a part of the fluidchamber; and an electrical current generating means for generating anelectrical current in the electrically conductive fluid in the part ofthe fluid chamber provided with the magnetic field, the actuation meansbeing further configured to provide a maintenance pulse for retractingthe meniscus of the electrically conductive fluid into the fluidchamber.